1. Field of the Invention
This invention relates to an optical recording medium having a wobbled groove, more particularly, to a novel optical recording medium capable of recording signals to a high density. The invention also relates to a method and apparatus for recording and/or reproducing such optical recording medium.
2. Description of the Related Art
The CD-R disc, used in, for example, a so-called compact disc recordable system (CD-R), has a wobbled groove. The sector information, inclusive of the address information, is recorded by modulating the wobbled signal.
Specifically, in a CD-R recording and/or reproducing apparatus, the wobbled signal having 22 kHz as carrier wave, is detected by a recording and/or reproducing light spot converged on the groove. A data string including the address information is detected by FM demodulating the wobbled signal.
In a system in which the address is arrayed at the leading end of the sector, the address information and the recording information are recorded time-divisionally, so that the recorded signals become discontinuous signals. With the present system, the data can be recorded continuously. Such feature is meritorious for an application in which emphasis is placed on interchangeability with the read-only disc on which the signals are recorded continuously.
In a method of recording the address information by modulating the wobbled signal, if the track pitch, which is the distance between neighboring groves, is reduced, the leakage of the wobbled signal from the neighboring groove is increased, thus lowering the S/N ratio of the wobbled signal. The address information not only cannot be demodulated correctly, but also the carrier wave of the wobbled signal, required for rotational control of the disc, becomes difficult to detect thus giving rise to obstructions in rotational control of the disc.
Since it is necessary to reduce the track pitch for recording the signal to a high density, it becomes necessary to reproduce the address information correctly despite the narrow track pitch.
Also, in the above system, the recording and/or reproducing spot on the disc derived from the reproduced address information depends as to its position accuracy on the frequency of the carrier wave and is substantially on the order of the wavelength of the carrier wave. On the other hand, the frequency of the carrier wave, that is the wobbling frequency, needs to be selected to a lower value in order to avoid adverse effects on the recording signal. In the case of the CD-R, the wobbling frequency is 22 kHz, with the wavelength on the disc being 54 .mu.m.
If data is recorded discontinuously, that is with interruptions, and data is recorded subsequently in the non-recorded portion, it is necessary to record data in correct positions on the disc. If correct recording is not possible, a so-called gap needs to be provided for absorbing errors in recording positions from one recording data unit to another for avoiding overlapping between recording data.
Since the gap reduces the recording capacity on the disc, the gap length needs be reduced to as small a value as possible. However, the above-mentioned accuracy is not sufficient.